The disclosure relates generally to turbomachine blades, and more particularly, to a blade having corrugated outer surface(s) to assist wake mixing and an internal rib having a corrugated surface to assist in cooling.
Turbomachine blades include airfoils that accelerate flow through contraction of area and the introduction of tangential velocity. The relative flow velocity exiting, for example, a gas turbine airfoil is quite high, typically with Mach numbers of 0.5 or higher. The finite thickness of an airfoil trailing edge, however, creates a velocity deficit, i.e., a wake, which introduces losses in the flow through viscous mixing. FIG. 1 shows an example of a typical unsteady loss process for a turbine blade row 10 operating behind a turbine vane row 12. At location 14, a wake is generated by a finite trailing edge thickness of the airfoil of vane row 12, resulting in aerodynamic losses due to mixing of the wake with the mainstream. At location 16, the wake interacts with potential field of a downstream airfoil of blade row 10, and it begins to distort. At location 18, the wake is segregated into discrete packages by the leading edge of airfoils in blade row 10. At location 20, a pressure gradient in the airfoil passage (between blades of blade row 10) causes wake packets to stretch and migrate, causing aerodynamic losses due to mixing of the wake packets (referred to as “free stream mixing”). That is, when the wake is ingested into a downstream airfoil of blade row 10, the wake undergoes a stretching and dilation process that exacerbates the losses associated with the mixing. At location 22, the wake packets interact with the boundary layer of the blades in blade row 10 downstream of the airfoils' wake, causing higher aerodynamic losses (airfoil surface losses). Unsteady loss caused by this phenomenon is present in all turbomachinery in various forms.
In order to address the above challenges, blades having airfoils with enhanced wake mixing structures have been proposed. The wake mixing structures can take a variety of forms such as crenulated or serrated trailing edges on the airfoils. These structures, however, are limited in their applicability because they must be formed or machined into the airfoil surface, which is a difficult and expensive process.
In addition to wake mixing, combustion or gas turbine engines (hereinafter “gas turbines”) include blades that must be actively cooled. In particular, gas turbines include a compressor, a combustor, and a turbine. As is well known in the art, in gas turbines, air compressed in the compressor is mixed with fuel and ignited in the combustor and then expanded through the turbine to produce power. The components within the turbine, particularly the circumferentially arrayed rotor and stator blades, are subjected to a hostile environment characterized by the extremely high temperatures and pressures of the combustion products that are expended therethrough. In order to withstand the repetitive thermal cycling as well as the extreme temperatures and mechanical stresses of this environment, the airfoils must have a robust structure and be actively cooled.
As will be appreciated, turbine rotor and stator blades often contain internal passages or circuits that form a cooling system through which a coolant, typically air bled from the compressor, is circulated. Such cooling circuits are typically formed by internal ribs that provide the required structural support for the airfoil, and include multiple flow path arrangements to maintain the airfoil within an acceptable temperature profile. The air passing through these cooling circuits often is vented through film cooling apertures formed on the leading edge, trailing edge, suction side, and/or pressure side of the airfoil.
It will be appreciated that the efficiency of gas turbines increases as firing temperatures rise. Because of this, there is a constant demand for technological advances that enable blades to withstand ever higher temperatures. These advances sometimes include new materials that are capable of withstanding the higher temperatures, but just as often they involve improving the internal configuration of the airfoil to enhance the blade's structure and cooling capabilities. However, because the use of coolant decreases the efficiency of the engine, new arrangements that rely too heavily on increased levels of coolant usage merely trade one inefficiency for another. As a result, there continues to be demand for new airfoil arrangements that offer internal airfoil configurations and coolant circulation that improves coolant efficiency.
A consideration that further complicates arrangement of internally cooled airfoils is the temperature differential that develops during operation between the airfoil's internal and external structure. That is, because they are exposed to the hot gas path, the external walls of the airfoil typically reside at much higher temperatures during operation than many of the internal ribs, which, for example, may have coolant flowing through passages defined to each side of them. In fact, a common airfoil configuration includes a “four-wall” arrangement in which lengthy inner ribs run parallel to the pressure and suction side outer walls. It is known that high cooling efficiency can be achieved by the near-wall flow passages that are formed in the four-wall arrangement. A challenge with the near-wall flow passages is that the outer walls experience a significantly greater level of thermal expansion than the inner walls. Various rib configurations have been devised to address these challenges.